XP dual boot on dual HD.. again

I shared my home PC with my 5 years son, and I got a lot of trouble since he always tried click anything. So, I decided to install new second harddisk, and I set a XP dual boot, one named DAD (older HD) and one named SON (new HD). I read in threads how to set dual boot, and there was no problem at all. But I found, even I have 2 XP on 2 hard disk, they still can access each other. If I boot on SON, I still acess (or modify) data in my old HD, vice versa. Is there any way to set HDs unaccessible each other (totally independent)? Or, maybe it works just like HD selector, that turn off one if we boot on other.
Tx very much.